Net Optical Gain Research Articles

Net Optical Gain from PbS Quantum Dot‐Doped Glasses under Continuous‐Wave Pumping

Net Optical Gain from PbS Quantum Dot‐Doped Glasses under Continuous‐Wave Pumping

Amplified spontaneous emission from inclusions containing cesium lead bromide in glasses.

Cesium lead halide (CsPbX3, X = Cl, Br and I) perovskite nanocrystals embedded glasses exhibit good optical properties and have potential as gain media. However, origins of the amplified spontaneous emission (ASE) from CsPbX3 nanocrystals are controversial. Here, it is found that ASE is from CsPbX3 nanocrystals in inclusions instead of CsPbX3 nanocrystals dispersed in the glass matrix. Inclusions with various sizes are capable of generating ASE, and ASE of the inclusions can sustain at energy densities as high as several tens of mJ/cm2. Thresholds of the fs laser energy densities increase with the increase in fs laser wavelength, and high net optical gain coefficient is obtained.

Electrically driven amplified spontaneous emission from colloidal quantum dots

Colloidal quantum dots (QDs) are attractive materials for realizing solution-processable laser diodes that could benefit from size-controlled emission wavelengths, low optical-gain thresholds and ease of integration with photonic and electronic circuits1–7. However, the implementation of such devices has been hampered by fast Auger recombination of gain-active multicarrier states1,8, poor stability of QD films at high current densities9,10 and the difficulty to obtain net optical gain in a complex device stack wherein a thin electroluminescent QD layer is combined with optically lossy charge-conducting layers11–13. Here we resolve these challenges and achieve amplified spontaneous emission (ASE) from electrically pumped colloidal QDs. The developed devices use compact, continuously graded QDs with suppressed Auger recombination incorporated into a pulsed, high-current-density charge-injection structure supplemented by a low-loss photonic waveguide. These colloidal QD ASE diodes exhibit strong, broadband optical gain and demonstrate bright edge emission with instantaneous power of up to 170 μW.

Stimulated Rayleigh Scattering Enhanced by a Longitudinal Plasma Mode in a Periodically Driven Dirac Semimetal Cd_{3}As_{2}.

Using broadband (12-45THz) multi-terahertz spectroscopy, we show that stimulated Rayleigh scattering dominates the transient optical conductivity of cadmium arsenide, a Dirac semimetal, under an optical driving field at 30THz. The characteristic dispersive line shape with net optical gain is accounted for by optical transitions between light-induced Floquet subbands, strikingly enhanced by the longitudinal plasma mode. Stimulated Rayleigh scattering with an unprecedentedly large refractive index change may pave the way for slow light generation in conductive solids at room temperature.

Optical net gain measurement on Al0.07Ga0.93N/GaN multi-quantum wells.

We report net gain measurements at room temperature in Al0.07Ga0.93N/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm-1, obtained at the maximum pumping power density of the experimental setup (743 kW/cm2). The net gain threshold was attained at 218 kW/cm2 using 193 nm optical pumping. From these experiments, we predict an electron-beam-pumped lasing threshold of 370 kW/cm2 at room temperature, which is compatible with the use of compact cathodes (e.g. carbon nanotubes). In some areas of the sample, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 µm) and high pumping power (superior to 550 kW/cm2), leading to an overestimation of the net gain value. We attribute such a phenomenon to the optical feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers.

State Filling and Stimulated Emission by Colloidal InP/ZnSe Core/Shell Quantum Dots

AbstractColloidal InP‐based quantum dots (QDs) are widely studied for luminescent color conversion or electroluminescence, yet the nature of the emitting state remains a matter of debate and reports on stimulated emission by these materials are nearly absent. Here, the properties of photo‐excited InP/ZnSe QDs are investigated using femtosecond transient absorption spectroscopy. It is shown that the evolution of the band‐edge bleach with increasing exciton number can be interpreted as state filling of the conduction‐ and valence‐band edge states by delocalized electrons and holes. In line with this interpretation, net stimulated emission is observed once the average exciton number exceeds 1. The authors account for this lower‐than‐expected gain threshold and the spectral properties of the gain band by reckoning that the Stokes shift between band‐edge absorption and emission is the dominant spectral shift. The underlying exciton–phonon coupling leads to a stimulated emission mechanism, where also single excitons can lead to net optical gain. To fully profit from this advantageous stimulated emission scheme, it is argued that InP/ZnSe QDs with more narrow emission lines are needed and spurious trapping of electron–hole pairs at high exciton numbers must be suppressed, for example, by better controlling the composition of the core/shell interface.

Ultrafast Antisolvent Growth of Single-Crystalline CsPbCl3 Microcavity for Low-Threshold Room Temperature Blue Lasing.

All-inorganic perovskite CsPbCl3 has recently attracted considerable attention due to its great potentials for the development of high-efficiency, deep-blue optoelectronic devices. Particularly, single-crystalline CsPbCl3 planar microstructures provide good platforms for both fundamental studies and nanophotonics applications from lasers and detectors to amplifiers. In this study, we report an ultrafast antisolvent deposition route to fabricate single-crystalline CsPbCl3 microplatelets (MPs). The as-grown MPs exhibit uniform morphology, strong emission, and outstanding gain property. Room temperature photoluminescence lasing is realized at 428 nm with a low threshold of 11.5 μJ cm-2 and high net optical gain up to 720 cm-1. These findings advance fundamental understanding on the fabrication and optoelectronic applications of low-dimensional CsPbCl3 perovskite structures.

Deep blue photoluminescence and optical gain from sodium-doped carbon dots

The photoluminescence (PL) from carbon dots can be dramatically modified through the engineering of surface oxygen-containing functional groups. Despite the recent surge of interest in the non-metal doping of carbon dots, the photo-physical properties of metal-doped carbon dots remain unexplored. Here, we report the highly efficient deep blue PL and amplified spontaneous emission from sodium (Na)-ion-doped carbon dots. Based on the structure- and temperature-dependent PL and the temporal evolution of PL, we reveal that the introduction of Na ions into carbon dots not only passivate the electronegative oxygen related defects but also create new Na–O related defect centers in the band gap, leading to deep blue PL with dramatically reduced bandwidth and significantly enhanced quantum yield. We establish a luminescence model with four energy levels to interpret the PL dynamics of Na+-doped carbon dots. We demonstrate for the first time the ASE from such carbon dots, which exhibit a net optical gain coefficient of 47.8 cm−1 at 440 nm. Our results open new horizons for improving the PL of carbon dots and pave the way for realizing optoelectronic devices based on carbon dots.

Net Optical Gain Coefficients of Cu+ and Tm3+ Single-Doped and Co-Doped Germanate Glasses.

Broadband tunable solid-state lasers continue to present challenges to scientists today. The gain medium is significant for realizing broadband tunable solid-state lasers. In this investigation, the optical gain performance for Tm3+ and Cu+ single-doped and co-doped germanate glasses with broadband emissions was studied via an amplified spontaneous emission (ASE) technique. It was found that the net optical gain coefficients (NOGCs) of Tm3+ single-doped glass were larger than those for Cu+ single-doped glass. When Tm3+ was introduced, the emission broadband width of Cu+-doped glass was effectively extended. Moreover, it was found that for the co-doped glass the NOGCs at the wavelengths for Tm3+ and Cu+ emissions were larger than those of Tm3+ and Cu+ single-doped glasses at the same wavelengths. In addition, the NOGC values of Tm3+ and Cu+ co-doped germanate glasses were of the same order of magnitude, and were maintained in a stable range at different wavelengths. These results indicate that the Tm3+ and Cu+ co-doped glasses studied may be a good candidate medium for broadband tunable solid-state lasers.

Observation of waveguide Fabry-Perot lasing in highly efficient Si nanocrystals

A luminescent Si nanocrystal (SiNC) thin film with photoluminescence quantum yield (PLQY) > 70% was made by using hydrogen silsesquioxane (HSQ), followed by long-time high-pressure hydrogenation. A net optical gain of 524 ± 21 cm−1 was obtained by means of variable stripe length-shifting excitation spot (VSL-SES). A rectangularly shaped SiO2 Fabry-Perot (F-P) cavity with size of 1.5 μm (width) × 0.7 μm (height) × 2000 μm (length) was made on top of the SiNC thin film. The waveguide F-P device was pumped with a 400 nm femtosecond pulsed laser. A threshold behavior of the light emission intensity as a function of the pumping power was observed. Other lasing characteristics including spectral narrowing, polarization of the emission, and small emission angle were also observed beyond the threshold pumping power.

Room-temperature Near-infrared Excitonic Lasing from Mechanically Exfoliated InSe Microflake.

The development of chip-level near-infrared laser sources using two-dimensional semiconductors is imperative to maintain the architecture of van der Waals integrated optical interconnections. However, the established two-dimensional semiconductor lasers may have either the disadvantages of poor controllability of monolayered gain media, large optical losses on silicon, or complicated fabrication of external optical microcavities. This study demonstrates room-temperature near-infrared lasing from mechanically exfoliated γ-phase indium selenide (InSe) microflakes free from external optical microcavities at a center wavelength of ∼1030 nm. The lasing action occurs at the sub-Mott density level and is generated by exciton-exciton scattering with a high net modal optical gain of ∼1029 cm-1. Moreover, the lasing is sustained for microdisks fabricated by a simple laser printing with a reduced threshold. These results suggest that InSe is a promising material for near-infrared microlasers and can be employed in a wide range of applications, including imaging, sensing, and optical interconnects.

Suppression of photo-induced effects in chemically stoichiometric Ge26.67Ga8S65.33 glasses

We have prepared GexGa8S92-x glasses with x=20, 26.67 and 36, and investigated the photoinduced effects under illumination at sub-bandgap wavelength with different power densities. It was found that, Ge20 and Ge36 undergo photodarkening (PD) and photobleaching (PB), respectively, and the change of transmission ratio with and without illumination increases with increasing illumination power density as well as prolonging illumination time. On the other hand, Ge26.67 is almost optical stable in any cases. This potentially offers a chance to reduce additional optical loss induced by PD and achieve net optical gain in the erbium doped chalcogenide planar waveguide amplifier using chemically stoichiometric Ge26.67 glass.

High Optical Gain of Solution‐Processed Mixed‐Cation CsPbBr3 Thin Films towards Enhanced Amplified Spontaneous Emission

AbstractA solution‐processed thin film made of all‐inorganic CsPbBr3 perovskite is a promising candidate for low‐cost and flexible green‐color lasers. However, the amplified spontaneous emission (ASE) of solution‐processed CsPbBr3 films still experiences a high threshold owing to poor morphology and insufficient optical gain. Here, a multiple‐cation doping strategy is demonstrated to develop compact, smooth thin films of Cs0.87(FAMA)0.13PbBr3/(NMA)2PbBr4 (FA: formamidinium; MA: methylammonium; NMA: naphthylmethylammonium) with a record high net modal optical gain of ≈3030 cm−1 and low propagation loss of 1.0 cm−1. The FA and MA cations improve the crystallization kinetics to form continuous films, and the NMA cations reduce the grain dimension, increase film dispersibility/uniformity, and enhance spatial confinement to promote optical gain. Room‐temperature ASE is demonstrated under a low threshold of ≈3.8 µJ cm−2 without degradation after four months of storage in glove box or excitation by 3 × 107 laser pulses. These findings provide insights into enhancing the optical gain and lowering the threshold of perovskite lasers in terms of molecular synthesis and microstructure engineering.

Optical gain and absorption of 1.55 μm InAs quantum dash lasers on silicon substrate

This Letter reports on the temperature-dependent optical gain and absorption features, including the quantum confined Stark effect, of an InAs/InGaAs quantum-dash laser directly grown on a (001) Si substrate, with the lasing wavelength within the range of 1.5–1.6 μm. The maximum optical net gain was 22 cm−1, and the internal optical loss was ∼–17 cm−1 at 20 °C. Measurements as a function of injection level indicate that while the required current densities are still high, the intrinsic performance is significantly better than that of similarly operated InAs quantum dots operating at 1.3 μm, and further effort on growth could be made to reduce the internal optical losses and non-radiative current density. Optical modal absorption spectra were measured as a function of reverse bias from 0 V to 6 V, and a 40 nm redshift was observed in the absorption edge due to the quantum confined Stark effect, suggesting potential applications of these materials in electro-absorption modulators grown on silicon.

Erbium-doped hybrid waveguide amplifiers with net optical gain on a fully industrial 300 mm silicon nitride photonic platform.

Recently, erbium-doped integrated waveguide devices have been extensively studied as a CMOS-compatible and stable solution for optical amplification and lasing on the silicon photonic platform. However, erbium-doped waveguide technology still remains relatively immature when it comes to the production of competitive building blocks for the silicon photonics industry. Therefore, further progress is critical in this field to answer the industry's demand for infrared active materials that are not only CMOS-compatible and efficient, but also inexpensive and scalable in terms of large volume production. In this work, we present a novel and simple fabrication method to form cost-effective erbium-doped waveguide amplifiers on silicon. With a single and straightforward active layer deposition, we convert passive silicon nitride strip waveguide channels on a fully industrial 300 mm photonic platform into active waveguide amplifiers. We show net optical gain over sub-cm long waveguide channels that also include grating couplers and mode transition tapers, ultimately demonstrating tremendous progress in developing cost-effective active building blocks on the silicon photonic platform.

Mode selection and high-quality upconversion lasing from perovskite CsPb2Br5 microplates

In recent years, halide perovskite nanostructures have had great advances and have opened up a bright future for micro/nanolasers. However, upconversion lasing by two-photon excitation with mode selection and high quality factor in one device is still rarely reported. Herein, two lasing modes are demonstrated in the all-inorganic perovskite CsPb 2 Br 5 microplates with subwavelength thickness and uniform square shape. The net optical gain is quickly established in less than 1 ps and persists more than 30 ps, revealed by ultrafast transient absorption spectroscopy. The temperature-dependent low-threshold amplified spontaneous emission confirms the net gain for stimulated emission with a high characteristic temperature of 403 K, far surpassing the all-inorganic CsPbBr 3 semiconductor gain media. Remarkably, upconversion lasing based on two kinds of microcavity effects, Fabry–Pérot and whispering-gallery modes, from the microplates at room temperature is successfully achieved with a low threshold operating in multi- or single-mode, respectively. Surprisingly, the quality factor ( ∼ 3551 ) is among the best values obtained from perovskite micro/nanoplate upconversion lasers without an external cavity. Moreover, the highly stable chromaticity with color drift only less than 0.1 nm also outbalances the all-inorganic CsPbBr 3 ones. These superior performances of CsPb 2 Br 5 microplate lasing with a facile solution synthesis procedure will offer a feasible structure to fabricate specific functionalities for high-performance frequency upconversion micro/nanoscale photonic integrated devices.

High-Gain Er3+:Al2O3 On-Chip Waveguide Amplifiers

In this work, high-gain on-chip Er3+ doped Al2O3 single-mode ridge waveguide amplifier device is reported. The growth of the active layer is realized using atomic layer deposition (ALD) compatible with Si processing technology. Optimization of the Al2O3 and Er2O3 growth conditions for fine-tuning the optical properties is systematically investigated. In order to optimize the active ion concentration, heterocycle structures consisting of separately optimized Al2O3 and Er2O3 are deposited. The structural and optical properties of the active layers were characterized by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, time-resolved photoluminescence, and optical gain measurements. We demonstrated up to 2.74 ± 0.40 dB total gain and 13.71 ± 1.97 dB/cm internal optical net gain per unit length at 1550 ± 12 nm using a 2-mm-long device, pumped using a 980 nm laser diode at 23-mW. This work can be expanded to other rare-earth ion doping processes, paving the way to significant improvements of active on-chip applications.

Redefinition the quasi-Fermi energy levels separation of electrons and holes inside and outside quantum wells of GaN based multi-quantum-well semiconductor laser diodes

Both accurate experiments and a self-consistent theoretical simulation using the near-ABC or ab initio method have uncovered the anomalous changes in the junction voltage (Vj ) of GaN-based lasers (Li et al 2013 Appl. Phys. Lett. 102 123501, Feng et al 2018 J. Phys. D: Appl. Phys. 51 095102 and Feng et al Appl. Phys. B 124 39). Here, further accurate emission spectral characteristics and band-gap simulation of a GaN based laser diodes (LDs) confirmed that previous researchers may have confused the junction voltage (Vj ) and the separation between the two Fermi levels of electrons and holes in the active region divided by the electronic charge (ΔEf /e). For the multiple-quantum wells (MQWs), there are obvious differences between them. For an ideal GaN-based LDs, despite the increase in junction voltage, physical quantities including the separation between the two Fermi levels of electrons and holes in the MQW region, and the optical net gain remain unchanged beyond the threshold region after an unpredictable sharp increase in the threshold region.

Net optical parametric gain in a submicron silicon core fiber pumped in the telecom band

A silicon core fiber (SCF) has been designed and fabricated with a dispersion engineered profile to support broadband optical parametric amplification across the telecom window. The combination of low optical transmission losses and high coupling efficiency of the SCF platform has allowed for an on-off optical parametric gain up to 9 dB, without experiencing gain saturation due to nonlinear absorption, resulting in a net off-waveguide gain of ∼2 dB. The ability to splice the SCFs with conventional silica fiber systems opens a route to compact and robust all-fiber integrated optical parametric amplifiers and oscillators that could find use in telecoms systems.

Tailoring optical properties and stimulated emission in nanostructured polythiophene

Polythiophenes are the most widely utilized semiconducting polymers in organic electronics, but they are scarcely exploited in photonics due to their high photo-induced absorption caused by interchain polaron pairs, which prevents the establishment of a window of net optical gain. Here we study the photophysics of poly(3-hexylthiophene) configured with different degrees of supramolecular ordering, spin-coated thin films and templated nanowires, and find marked differences in their optical properties. Transient absorption measurements evidence a partially-polarized stimulated emission band in the nanowire samples, in contrast with the photo-induced absorption band observed in spin-coated thin films. In combination with theoretical modeling, our experimental results reveal the origin of the primary photoexcitations dominating the dynamics for different supramolecular ordering, with singlet excitons in the nanostructured samples superseding the presence of polaron pairs, which are present in the disordered films. Our approach demonstrates a viable strategy to direct optical properties through structural control, and the observation of optical gain opens the possibility to the use of polythiophene nanostructures as building blocks of organic optical amplifiers and active photonic devices.